The embodiments disclosed herein relate to heaters and more particularly to the efficient design and operation of such heaters.
The steam cracking or pyrolysis of hydrocarbons for the production of olefins is often carried out in tubular coils located in fired heaters. The pyrolysis process is considered to be the heart of an olefin plant and has a significant influence on the economics of the overall plant.
The hydrocarbon feedstock may be any one of the wide variety of typical cracking feedstocks such as methane, ethane, propane, butane, mixtures of these gases, naphthas, gas oils, etc. The product stream contains a variety of components; the concentrations of these components are dependent in part upon the feed selected. In a conventional pyrolysis process, vaporized feedstock is fed together with dilution steam to a tubular reactor located within the fired heater. The quantity of dilution steam required is dependent upon the feedstock selected; lighter feedstocks such as ethane require lower steam (0.2 lb./lb. feed), while heavier feedstocks such as naphtha and gas oils require steam/feed ratios of 0.5 to 1.0. The dilution steam has the dual function of lowering the partial pressure of the hydrocarbon and reducing the fouling rate of the pyrolysis coils.
Fouling on the inside surface of the radiant pyrolysis coils is one of the determining factors for the onstream time of these heaters. As the time of operation increases, the buildup of coke creates a resistance to heat transfer from the radiant firebox. In order to maintain constant process performance, as exemplified by a constant outlet temperature of the coil, the heat flux to the coil must be maintained. The coke layer on the inside of the coil acts as a resistance to heat flux and the outside metal temperature of the tube must increase to allow for the equivalent flux through a higher resistance. The time that a heater can operate before a shutdown to remove the coke deposits depends on two primary factors. The first is the rate of fouling. Fouling occurs as coke builds up on the radiant heating coil. As coke is deposited on the coil, it inhibits the transfer of heat from the coil. As a result, the buildup of coke requires more heat to be added to the system to maintain the efficiency of the heater. The rate of fouling is a function of process load (heat flux required), dilution steam, temperature at the metal surface on the inside of the coil, and the characteristics of the feedstock itself. For example, heavier feeds coke faster than lighter feeds. It is desired to maximize the onstream time.
The second factor is the makeup of the radiant heating coil. Typically, the coil is made up of a metal or metal alloy. Metals and alloys are sensitive to extreme temperatures. That is, if the radiant coil is exposed to a temperature above its maximum mechanical threshold, it will begin to deteriorate, causing damage to the radiant heating coil. As a result, a typical pyrolysis heater must be carefully monitored to maintain specific temperature ranges. This become problematic as coke builds up on the coil because more heat must be added to maintain the efficiency of the system.
As a result, it is desirable to design pyrolysis coils with long cycle times to minimize the maximum tube metal temperatures while maximizing the total heat transferred through the coil. This allows for the maximum temperature rise at a constant fouling rate.
In a typical pyrolysis process, the steam/feed mixture is preheated to a temperature just below the onset of the cracking reaction, which is usually about 600° C. This preheat occurs in the convection section of the heater. The mix then passes to the radiant section where pyrolysis reactions occur. Generally the residence time in the pyrolysis coil is in the range of 0.2 to 0.4 seconds and outlet temperatures for the reaction are on the order of about 700° to 900° C. The reactions that result in the transformation of saturated hydrocarbons to olefins are highly endothermic thus requiring high levels of heat input. This heat input must occur at elevated reaction temperatures. It is generally recognized in the industry that for most feedstocks, and especially for heavier feedstocks such as naphtha, shorter residence times will lead to higher selectivity to ethylene and propylene since secondary degradation reactions will be reduced. Further it is recognized that the lower the partial pressure of the hydrocarbon within the reaction environment, the higher the selectivity.
The flue gas temperatures in the radiant section of the fired heater are typically above 1,100° C. The heat transfer to the coils is primarily by radiation. In some conventional designs, approximately 32 to 40% of the heat fired as fuel into the heater is transferred into the coils in the radiant section. The balance of the heat is recovered in the convection section either as feed preheat or as steam generation. Given the limitation of small tube volume to achieve short residence times and the high temperatures of the process, heat transfer into the reaction tube is difficult. As a result, high heat fluxes are used and the operating tube metal temperatures are close to the mechanical limits for even exotic metallurgies. In most cases, tube metal temperatures limit the extent to which residence time can be reduced as a result of a combination of higher process temperatures required at the coil outlet and the reduced tube length (hence tube surface area) which results in higher flux and thus higher tube metal temperatures. Tube metal temperatures are also a limiting factor in determining the capacity of these radiant coils since more flux is required for a given tube when operated at higher capacity. The exotic metal reaction tubes located in the radiant section of the cracking heater represent a substantial portion of the cost of the heater so it is important that they be utilized fully. Utilization is defined as operating at as high and as uniform a heat flux as possible consistent with the design objectives of the heater. This will minimize the number and length of the tubes and the resulting total metal surface area required for a given pyrolysis capacity.
In a typical cracking furnace, the heat is supplied by a combination of hearth and wall burners. The pyrolysis coils are typically suspended from the top of the radiant section and hang between two radiant walls. The hearth and wall burner combination heats the walls of the furnace that then radiate to the coils. A small portion of the heat transferred is done convectively by the flue gases within the firebox transferring the heat directly to the coils. However in a typical furnace, greater than 85% of the heat is transferred radiatively. Hearth burners are installed in the floor of the firebox and fire vertically up along the walls. Wall burners are located in the vertical walls of the furnace and fire radially out along the walls.
In any flame from a burner, there is a characteristic combustion profile. As the fuel and air mixture leaves the burner, combustion begins. As the combustion reaction continues, the temperature of the combustion mixture increases and heat is released. At some distance from the burner, there is a point of maximum combustion and hence maximum heat release. During this process, heat is absorbed by the process coil. The characteristics of the flame depend upon the total firing from that burner and the specifics of the burner design. Different flame shapes and heat release profiles are possible, depending upon how the fuel and air are mixed. Hearth burners typically operate at a fired duty between about 5 and 15 MM BTU/hr. In these burners, the point of maximum combustion is typically about 3 to 4 meters above the burner itself. Because of the characteristic heat release profile from these burners, an uneven heat flux profile (heat absorbed profile) is sometimes created. The typical flux profile for the radiant coil shows a peak flux near the center elevation of the firebox (at the point of maximum combustion or heat release for the hearth burners) with the top and bottom portions of the coil receiving less flux. In some heaters, radiant wall burners are installed in the top part of the sidewalls to equalize the heat flux profile in the top portion of the coil. Typical coil surface heat flux profiles and metal temperature profiles for a hearth burner and for a combination of hearth and wall burners at the same heat liberation rate show low heat flux and metal temperature in the lower portion of the firebox, which means that the coil in this portion may be underutilized.
There have been a number of attempts to control the flux profile within a pyrolysis heater. It is known that staging the fuel to hearth burners can be used to adjust the flame shape and thus impact the point of maximum heat release. Hearth burners are typically designed with several differing fuel injection points. Air is drawn into the furnace via either by natural or induced draft or by inspiration with fuel utilizing a venturi system. A primary fuel is injected into this air stream with the purpose of providing sufficient combustion to develop a stable flame. In some cases another small fuel injection point is used just adjacent to this primary flame to help stabilize the flame and prevent flame blowout. Older hearth burners typically feed 100% of the hearth burner fuel fired with these primary fuel injection points. The combustion occurred at an air to fuel ratio of slightly above stoichiometric (10-15% excess air).
When NOx values are an important consideration, some of the fuel from the primary injection point can be removed from the entering air flow and placed in secondary or staged tips just at the edge of the burner. This fuel is directed such that it will mix with the flowing air and primary fuel stream at some distance above the burner. By “staging” the mixing of fuel and air, the combustion profile of the flame can be altered, leading to a lower flame temperature and hence lower NOx. This technique also changes the point of maximum combustion and thus impacts the resultant flux profile to the coil. Staging the fuel does not change the net air to fuel ratio of the burner, it just changes when and where the fuel is mixed. The amount of secondary fuel injection, the location of that injection point at the edge of the burner, and the angle at which it is injected all impact the NOx values, flame shape, and hence the coil metal temperature profile.
U.S. Pat. No. 4,887,961 describes radiant wall burners in which air and fuel are pre-mixed in a venturi to proportions equivalent to 10-15% excess air. The venturi is sized to inspirate the correct amount of air using the fuel as the motive force in the throat of the venturi. In U.S. Pat. No. 6,796,790 a wall burner is described that takes part of the fuel and injects it just beyond the “can” or “deflector” and relies on fluid dynamics to pull this “secondary staged fuel—for wall burners” into the flow of 100% of the air and part of the fuel.
U.S. Pat. No. 6,616,442 describes a hearth burner with a first “zone” just above the burner where the mixture of fuel and air (excess air) leaves the tile and burns. The second “zone” is at a higher elevation where the secondary fuel mixes with the burning air/fuel mixture. The net resulting air to fuel mix at the second zone is slightly above a stoichiometric ratio.
Another means of controlling coil metal temperatures is described in U.S. Pat. No. 6,685,893. In this patent, a wall burner is specifically placed in the floor of the furnace and the flame is directed along the floor in order to heat the refractory floor of the furnace and provide additional radiation surface for the lower portion of the coil. The base burner can be designed to inspirate air and produce a slightly greater than stoichiometnc air to fuel mixture for combustion. Alternately the base burner can utilize fuel withdrawn from the secondary staged tips of the hearth burner. In order to have a stable flame from the base burner, some quantity of air is required to be fed with this fuel. Since the base burner is located in very close proximity to the hearth burner, there are a number of combinations of air and fuel for these separate burners that still result in a slightly greater than stoichiometric combustion mixture at or near the floor of the heater. The vertically firing hearth burner can operate with excess air and the base burner with a sub-stoichiometric amount of air or they can be operated in reverse with the base burner having excess air and the hearth burner with slightly sub-stoichiometric air. Some important design points are that by making the floor part of the radiant surface, the tube metal temperature can be reduced, and by staging the combustion through by staging of the fuel (and excess air location at the floor), NOx production can be reduced.
In U.S. Pat. No. 7,172,412, a different approach is used to control metal temperatures and flux profiles. Fuel is withdrawn from the secondary staged tips of the hearth burner and injected into the furnace at some distance above the hearth burner through the walls of the furnace. This injection serves to create a low pressure zone along the wall and thus the flame is “pulled” to the wall thus reducing proximity of the point of maximum combustion to the pyrolysis coil. Under these conditions, the hearth burner is operated under excess air conditions while the balance of the fuel is added through the wall at a point above the hearth burner. This approach not only stages the fuel to reduce NOx but alters the flame shape by pulling it back to the wall thus reducing metal temperatures.
Improving the hearth burner flux profile can be difficult because of NOx requirements and because of the steadily increasing demand for higher burner heat releases. Another way to equalize the flux profile is by using wall burners only. However, since the maximum heat release of a wall burner is about 10 times less than that of a hearth burner, the significant number of wall burners needed to generate an equivalent heat release profile limits the practicality of this approach.